1. Field of the Invention
The present invention generally relates to a heating device and method for cooking food or the like, and more particularly, to a high-frequency heating device and method for cooking food using microwaves, and to a heater element sheathed by dielectric material, such as a quartz-sheathed element heater or the like.
2. Description of the Prior Art
In some conventional high-frequency heating devices, a hollow choke damper is provided at a location where a pipe-shaped dielectric heater extends through a wall structure of a heating chamber. In some other conventional devices, a small shielding chamber for shielding electric waves is provided outside of the heating chamber. Accordingly, these devices are complicated in construction and have some problems.
In these devices, when heating by the dielectric heater is followed by heating by microwaves or when the former and the latter are alternately performed, a dielectric portion of the heater becomes high in temperature, thereby causing a dielectric loss to become large. Under such conditions, when the microwave heating is performed, a dielectric pipe is partially heated by the microwaves, thus occasionally causing the dielectric pipe to be damaged or heating wires constituting the heater to be cut off.
FIGS. 1 and 2 depict one of the above-described conventional heating devices.
As shown in FIGS. 1 and 2, a door 2 is hingedly connected to a housing of the device, in which a heating chamber 1 is formed. A magnetron 3 securely mounted in the housing emits electric waves into the heating chamber 1 through a waveguide 4 so that food 5 or the like may be heated by electric waves. A pair of hollow choke dampers 6 and 7 are cylindrically formed on opposite side walls of the heating chamber 1. A pipe 8 made of a heat-resistant dielectric such as quarts glass or the like extends through the heating chamber 1 and both the choke dampers 6 and 7. The pipe 8 accommodates a heating wire 9 having opposite ends connected to respective lead wires 10 and 11, which are lead out of the housing so that the heating wire 9 may be supplied with electricity via the lead wires 10 and 11.
FIG. 3 depicts one of the choke dampers 6 and 7.
Each end of the pipe 8 is supported by an insulator 14, and each of the choke dampers 6 and 7 comprises an internal wall 12 and an external wall 13 rigidly secured to each other. A recess defined by the internal and external walls 12 and 13 has a length X approximately equal to odd multiples of a quarter-wavelength .lambda./4 of electric waves to be used, thereby enabling high-frequency electric waves to be transmitted along the pipe 8, the lead wire 10 and the internal wall 12. Accordingly, protection against the leakage of electric waves is achieved by preventing the electric waves from being led out of the housing via the pipe 8 and the lead wire 10.
In such a construction, however, the internal configuration of the housing becomes complicated, since the hollow choke dampers 6 and 7 must be provided on internal walls of the heating chamber 1, through which the pipe 8 extends. This fact undesirably increases the cost of manufacture of the heating device. There is also another problem in that the radiating surface of the heating wire 9 inside the pipe 8 becomes short. As a result, the microwave heating acts extremely strongly on the dielectric pipe of the heater at locations a certain distance away from the internal walls of the heating chamber 1, in which openings for receiving the pipe 8 are formed.
The inventors of the instant application tried to arrange the choke dampers without any protrusion inside the heating chamber. In such an arrangement, upon application of high-frequency electric waves to the dielectric pipe of the heater, the exothermic conditions caused by the dielectric loss of the dielectric pipe were observed using a radiating thermometer or the like. As a result, a problem arose is that the microwave heating occasionally brought about partial high-temperature portions.
Furthermore, when the microwave heating was performed immediately after the heating by the heating wire 9, heat generated by the heating wire 9 increased the dielectric loss of the pipe 8 itself, thus causing partial abnormal heating. As a result, a problem occasionally arose in which the pipe 8 was melted or damaged or the heating wire 9 was cut off.